Multiple entry points for system call instructions

ABSTRACT

A processor executes a system call instruction. The processor includes at least two registers in which target addresses may be stored, and selects the target address from one of the registers responsive to the operating mode. Different target addresses may be programmed into the registers, and thus the operating mode of the code sequence may be indicated by which target address is selected.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention is related to the field of processors and, more particularly, to system call instructions in processor architectures.

[0003] 2. Description of the Related Art

[0004] The x86 architecture (also known as the IA-32 architecture) has enjoyed widespread acceptance and success in the marketplace. Accordingly, it is advantageous to design processors according to the x86 architecture. Such processors may benefit from the large body of software written to the x86 architecture (since such processors may execute the software and thus computer systems employing the processors may enjoy increased acceptance in the market due to the large amount of available software).

[0005] The x86 architecture supports a set of four privilege levels at which operating system and/or application code may execute. Application code typically executes at the lowest privilege level (3), and at least some of the operating system code typically executes at the highest privilege level (0). Changes in privilege level are accomplished using a relatively slow gate mechanism within the segmentation mechanism of the x86 architecture. In the gate mechanism, low privileged code (e.g. application code) may transfer control to higher privileged code through a gate descriptor in the segment descriptor table (which may be located by arguments to the CALL instruction supported in the x86 architecture) which includes protection checking to ensure the privilege transition is allowed and which points to yet another segment descriptor which locates the higher privileged code. Numerous protection checks are performed during the transition.

[0006] Unfortunately, the gate mechanism is too slow to allow for efficient calls to operating system services, reducing the execution performance of the system. To address this problem, Advanced Micro Devices, Inc. introduced two instructions, a system call (SYSCALL) instruction and a system return (SYSRET) instruction. These instructions allow for a low latency call to the operating system and return from the operating system which bypasses numerous protection checks and memory accesses to the descriptor tables. Intel Corp. also introduced SYSENTER and SYSEXIT instructions for similar reasons. The system call instruction branches to the operating system at an address specified in a register (e.g. a model specific register).

[0007] As computer systems have continued to evolve, 64 bit address size (and sometimes operand size) has become desirable. A larger address size allows for programs having a larger memory footprint (the amount of memory occupied by the instructions in the program and the data operated upon by the program) to operate within the memory space. A larger operand size allows for operating upon larger operands, or for more precision in operands. More powerful applications and/or operating systems may be possible using 64 bit address and/or operand sizes. Thus, it may be desirable to provide an architecture which is compatible with the x86 processor architecture but which includes support for 64 bit processing as well.

[0008] Supporting 64 bit processing may complicate operating system calls and returns. Generally, the application program may load operands into registers, predefined memory locations, and/or the stack prior to performing the system call instruction. The operands may specify the desired operating system service as well as operands for that service. Unfortunately, the size of the operands may depend on the operating mode in which the application program is executing (e.g. 32 bit applications may provide 32 bit operands and 64 bit applications may provide 64 bit operands). Thus, the location of each operand (e.g. in memory or on the stack) may depend on the operating mode of the application program. Furthermore, the operands in registers may require modification (e.g. sign extension) before being used if the application program is 32 bit. It may not be easy to quickly determine the operating mode of the application program.

SUMMARY OF THE INVENTION

[0009] A processor is described which executes a system call instruction. The processor includes at least two registers in which target addresses may be stored, and selects the target address from one of the registers responsive to the operating mode. Different target addresses may be programmed into the registers, and thus the operating mode of the code sequence may be indicated by which target address is selected. For example, instructions at each of the target addresses may translate the operands provided by the calling code sequence into a form used by the called code sequence, and then may branch to the code sequence. Since the calling code sequence's operating mode is known based on the target address selected in response to the system call instruction, instructions to determine the operating mode may be omitted, which may allow for improved performance.

[0010] Additionally, since multiple registers may be provided for storing target addresses for different operating modes, operating system design may be simplified. If only one register were provided, then that register would be updated each time a code sequence was launched which had a different operating mode. Task switching in multitasking operating systems would be more complex and time consuming due to the need to change the target address for the system call instruction each time a task switch occurs (or at least each time a task switch occurs between tasks which are operating in different operating modes).

[0011] Broadly speaking, a processor is contemplated which comprises a first register, a second register, and an execution core coupled thereto. The first register is configured to store a first target address. The second register is configured to store a second target address. The execution core is configured, responsive to a first instruction, to: (i) select the first target address from the first register as a next program counter address if a first operating mode is active in the processor, and (ii) select the second target address from the second register as the next program counter address if a second operating mode is active in the processor.

[0012] Additionally, an apparatus is contemplated comprising a first storage location, a second storage location, and a processor. The first storage location corresponds to a first register, and stores a first target address. The second storage location corresponds to a second register, and stores a second target address. Coupled to the first storage location and the second storage location, the processor is configured, responsive to a first instruction, to: (i) select the first target address from the first storage location as a next program counter address if a first operating mode is active, and (ii) select the second target address from the second storage location as the next program counter address if a second operating mode is active.

[0013] Moreover, a method is contemplated. A first target address is selected from a first register as a next program counter address responsive to a first operating mode during execution of a first instruction. A second target address is selected from a second register as the next program counter address responsive to a second operating mode during execution of the first instruction.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The following detailed description makes reference to the accompanying drawings, which are now briefly described.

[0015]FIG. 1 is a block diagram of one embodiment of a processor.

[0016]FIG. 2 is a block diagram of one embodiment of a segment descriptor for 32/64 mode.

[0017]FIG. 3 is a block diagram of one embodiment of a segment descriptor for compatibility mode.

[0018]FIG. 4 is a table illustrating one embodiment of operating modes as a function of segment descriptor and control register values.

[0019]FIG. 5 is a block diagram illustrating one embodiment of operand passing for a 32 bit caller and a 64 bit caller.

[0020]FIG. 5A is a block diagram illustrating exemplary 32 and 64 bit callers, entry point code, and operating system services.

[0021]FIG. 6 is a block diagram illustrating one embodiment of registers used by the system call and system return instructions.

[0022]FIG. 7 is a flowchart illustrating operation of one embodiment of an execution core shown in FIG. 1 during execution of a system call instruction.

[0023]FIG. 8 is a flowchart illustrating operation of one embodiment of an execution core shown in FIG. 1 during execution of a system return instruction FIG. 9 is a block diagram illustrating one embodiment of a system call instruction and one embodiment of a system return instruction.

[0024]FIG. 10 is a flowchart illustrating one embodiment of an interpreter.

[0025]FIG. 11 is a flowchart illustrating one embodiment of a translator.

[0026]FIG. 12 is a block diagram illustrating one embodiment of mapping non-native architected state.

[0027]FIG. 13 is a block diagram illustrating a second embodiment of mapping non-native architected state.

[0028]FIG. 14 is a block diagram illustrating a third embodiment of mapping non-native architected state.

[0029]FIG. 15 is a block diagram of one embodiment of a carrier medium.

[0030]FIG. 16 is a block diagram of one embodiment of a computer system including the processor shown in FIG. 1.

[0031]FIG. 17 is a block diagram of another embodiment of a computer system including the processor shown in FIG. 1.

[0032] While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] Processor Overview

[0034] Turning now to FIG. 1, a block diagram illustrating one embodiment of a processor 10 is shown. Other embodiments are possible and contemplated. In the embodiment of FIG. 1, processor 10 includes an instruction cache 12, an execution core 14, a data cache 16, an external interface unit 18, a memory management unit (MMU) 20, a register file 22, and a set of model specific registers (MSRs) 36. In the illustrated embodiment, MW 20 includes a set of segment registers 24, a first control register 26, a second control register 28, a local descriptor table register (LDTR) 30, a global descriptor table register (GDTR) 32, and a page table base address register (CR3) 34. Instruction cache 12 is coupled to external interface unit 18, execution core 14, and MMU 20. Execution core 14 is further coupled to MMU 20, register file 22, MSRs 36, and data cache 16. Data cache 16 is further coupled to MMU 20 and external interface unit 18. External interface unit 18 is further coupled to MW 20 and to an external interface.

[0035] Processor 10 may employ a processor architecture compatible with the x86 architecture (also known as the IA-32 architecture) and including additional architectural features to support 64 bit processing. More particularly, the processor architecture employed by processor 10 may define a mode, referred to below as “long mode”. Long mode is a mode in which 64 bit processing is selectable as an operating mode, as well as 32 bit or 16 bit processing as specified in the x86 architecture. More particularly, long mode may provide for an operating mode in which virtual addresses may be greater than 32 bits in size.

[0036] Processor 10 may implement a mechanism allowing for orderly transition to and from long mode, even though multiple registers may be changed to perform the transition. Particularly, processor 10 may employ a long mode active (LMA) indication in a control register (e.g. control register 26 in the present embodiment, although the LMA indication may be stored in any control register, including control registers not storing the LME indication). The processor 10 may use the LMA indication as the indication of whether or not long mode is active (i.e. whether or not the processor is operating in long mode). However, the LMA indication may not be modified directly via an instruction. Instead, an instruction is used to change the state of the LME indication to indicate whether or not long mode is desired. Long mode may be activated (as indicated by the LMA indication) via the combination of enabling paging (as indicated by the PG indication in control register 28 and described in more detail below) and the LME indication indicating that long mode is desired. Viewed in another way, the LME indication may be used to enable the transition to long mode. The LMA indication may indicate whether or not the transition has successfully occurred, and thus indicates whether processor 10 is operating according to the long mode definition or processor 10 is operating according to the legacy definition of the x86 processor architecture.

[0037] Processor 10 is configured to establish an operating mode in response to information stored in a code segment descriptor corresponding to the currently executing code and further in response to one or more enable indications stored in one or more control registers. As used herein, an “operating mode” specifies default values for various programmably selectable processor attributes. For example, the operating mode may specify a default operand size and a default address size. The default operand size specifies the number of bits in an operand of an instruction, unless an instruction's encoding overrides the default. The default address size specifies the number of bits in an address of a memory operand of an instruction, unless an instruction's encoding overrides the default. The default address size specifies the size of at least the virtual address of memory operands. As used herein, a “virtual address” is an address generated prior to translation through an address translation mechanism (e.g. a paging mechanism) to a “physical address”, which is the address actually used to access a memory. Additionally, as used herein, a “segment descriptor” is a data structure created by software and used by the processor to define a segment of memory and to further define access control and status for the segment. A “segment descriptor table” is a table in memory storing segment descriptors. Since there is more than one operating mode, the operating mode in effect at any given time may be described as being the “active” operating mode.

[0038] In the illustrated embodiment, MMU 20 generates an operating mode and conveys the operating mode to execution core 14. Execution core 14 executes instructions using the operating mode. More particularly, execution core 14 fetches operands having the default operand size from register file 22 or memory (through data cache 16, if the memory operands are cacheable and hit therein, or through external interface unit 18 if the memory operands are noncacheable or miss data cache 16) unless a particular instruction's encoding overrides the default operand size, in which case the overriding operand size is used. Similarly, execution core 14 generates addresses of memory operands, wherein the addresses have the default address size unless a particular instruction's encoding overrides the default address size, in which case the overriding address size is used. In other embodiments, the information used to generate the operating mode may be shadowed locally in the portions of processor 10 which use the operating mode (e.g. execution core 14), and the operating mode may be determined from the local shadow copies.

[0039] As mentioned above, MMU 20 generates the operating mode responsive to a code segment descriptor corresponding to the code being executed and further responsive to one or more values in control registers. Information from the code segment descriptor is stored in one of the segment registers 24 (a register referred to as CS, or code segment). Additionally, control register 26 stores an enable indication (LME) which is used to enable transition to long mode and the LMA indication indicating whether or not long mode is active. In long mode, an operating mode in which the default address size is greater than 32 bits (“32/64 mode”) as well as certain compatibility modes for the 32 bit and 16 bit operating modes may be available using the segment descriptor indications. The default operand size may be 32 bits in 32/64 mode, but instructions may override the default 32 bit operand size with a 64 bit operand size when desired. If the LME indication is in an enabled state, then long mode may be activated. If the LNE indication is in a disabled state, then long mode may not be activated. In one embodiment, the default address size in 32/64 mode may be implementation-dependent but may be any value up to and including 64 bits. Furthermore, the size of the virtual address may differ in a given implementation from the size of the physical address in that implementation.

[0040] It is noted that various indications are described herein (e.g. LMA, LME, etc.). Generally, an indication is a value which may be placed into two or more states. Each state may be assigned a meaning. Some of the indications described herein (including some enable indications) may be described as bits. The bit being set may be one state (e.g. the enabled state for enable indications) and the bit being clear may be the other state (e.g. the disabled state for enable indications). However, other encodings are possible, including encodings in which multiple bits are used and encodings in which the enabled state is the clear state and the disabled state is the set state. Accordingly, the remainder of this description may refer to the LME indication in control register 26 as the LME bit, with the enabled state being set and the disabled state being clear. However, other encodings of the LME indication are contemplated, as set forth above. Similarly, the LMA indication may be referred to as the LMA bit, with the set state indicating that long mode is active and the clear state indicating that long mode is inactive. However, other encodings of the LMA indication are contemplated, as set forth above.

[0041] Segment registers 24 store information from the segment descriptors currently being used by the code being executed by processor 10. As mentioned above, CS is one of segment registers 24 and specifies the code segment of memory. The code segment stores the code being executed. Other segment registers may define various data segments (e.g. a stack data segment defined by the SS segment register, and up to four data segments defined by the DS, ES, FS, and GS segment registers). FIG. 1 illustrates the contents of an exemplary segment register 24A, including a selector field 24AA and a descriptor field 24AB. Selector field 24AA is loaded with a segment selector to activate a particular segment in response to certain segment load instructions executed by execution core 14. The segment selector locates the segment descriptor in a segment descriptor table in memory. More particularly, processor 10 may employ two segment descriptor tables: a local descriptor table and a global descriptor table. The base address of the local descriptor table is stored in the LDTR 30. Similarly, the base address of the global descriptor table is stored in GDTR 32. A bit within the segment selector (the table indicator bit) selects the descriptor table, and an index within the segment selector is used as an index into the selected table. When an instruction loads a segment selector into one of segment registers 24, MMU 20 reads the corresponding segment descriptor from the selected segment descriptor table and stores information from the segment descriptor into the segment descriptor field (e.g. segment descriptor field 24AB for segment register 24A). The information stored in the segment descriptor field may comprise any suitable subset of the segment descriptor, including all of the segment descriptor, if desired. Additionally, other information derived from the segment descriptor or other sources may be stored in the segment descriptor field, if desired. For example, an embodiment may decode the operating mode indications from the code segment descriptor and store the decoded value rather than the original values of the operating mode indications. If an instruction-causes CS to be loaded with a segment selector, the code segment may change and thus the operating mode of processor 10 may change.

[0042] In one embodiment, only the CS segment register is used in 32/64 mode. The data segment registers are ignored from the standpoint of providing segmentation information. In 16 and 32 bit modes, the code segment and data segments may be active. Furthermore, a second enable indication (PE) in control register 28 may affect the operation of MMU 20. The PE enable indication may be used to enable protected mode, in which segmentation and/or paging address translation mechanisms may be used. If the PE enable indication is in the disabled state, segmentation and paging mechanisms are disabled and processor 10 is in “real mode” (in which addresses generated by execution core 14 are physical addresses). Similar to the LME indication, the PE indication may be a bit in which the enabled state is the bit being set and the disabled state is the bit being clear. However, other embodiments are contemplated as described above. Generally, a “protected mode” is a mode in which various hardware and/or software mechanisms are employed to provide controlled access to memory.

[0043] Control register 28 is further illustrated in FIG. 1 as storing a paging enable indication (PG). The PG indication may indicate whether or not paging is enabled. As mentioned above, the LMA bit is set once paging is enabled and the LME bit is set. As used herein, the term “paging” or “paging address translation” refers to the translation of virtual addresses to physical addresses using mappings stored in a page table structure indicated by the page table base address register 34. A given page mapping maps any virtual address having the same virtual page number to a corresponding physical address in a page of physical memory. The page table is a predefined table of entries stored in memory. Each of the entries store information used to map virtual addresses to physical addresses.

[0044] It is noted that MU 20 may employ additional hardware mechanisms, as desired. For example, MMU 20 may include paging hardware to implement paging address translation from virtual addresses to physical addresses. The paging hardware may include a translation lookaside buffer (TLB) to store page translations.

[0045] It is noted that control registers 26 and 28 may be implemented as architected control registers (e.g. control register 26 may be CR4 and control register 28 may be CR0). Alternatively, one or both of the control registers may be implemented as model specific registers within MSRs 36 to allow for other uses of the architected control registers without interfering with 32/64 mode. Generally, the control registers are each addressable by one or more instructions defined in the processor architecture, so that the registers may be changed as desired.

[0046] Instruction cache 12 is a high speed cache memory for storing instruction bytes. Execution core 14 fetches instructions from instruction cache 12 for execution. Instruction cache 12 may employ any suitable cache organization, including direct-mapped, set associative, and fully associative configurations. If an instruction fetch misses in instruction cache 12, instruction cache 12 may communicate with external interface unit 18 to fill the missing cache line into instruction cache 12. Additionally, instruction cache 12 may communicate with MMU 20 to receive physical address translations for virtual addresses fetched from instruction cache 12.

[0047] Execution core 14 executes the instructions fetched from instruction cache 12. Execution core 14 fetches register operands from register file 22 and updates destination registers in register file 22. The size of the register operands is controlled by the operating mode and any overrides of the operating mode for a particular instruction. Similarly, execution core 14 fetches memory operands from data cache 16 and updates destination memory locations in data cache 16, subject to the cacheability of the memory operands and hitting in data cache 16. The size of the memory operands is similarly controlled by the operating mode and any overrides of the operating mode for a particular instruction. Furthermore, the size of the addresses of the memory operands generated by execution core 14 is controlled by the operating mode and any overrides of the operating mode for a particular instruction. Execution core 14 may also fetch operands from or update MSRs 36 or and of the registers illustrated in MMU 20 in FIG. 1.

[0048] Execution core 14 may employ any suitable construction. For example, execution core 14 may be a superpipelined core, a superscalar core, or a combination thereof. Execution core 14 may employ out of order speculative execution or in order execution, according to design choice. Execution core 14 may include microcoding for one or more instructions or exception situations, in combination with any of the above constructions.

[0049] Register file 22 may include 64 bit registers which may be accessed as 64 bit, 32 bit, 16 bit, or 8 bit registers as indicated by the operating mode of processor 10 and any overrides for a particular instruction. The registers included in register file 22 may include the RAX, RBX, RCX, RDX, RDI, RSI, RSP, and RBP registers (which may be 64 bit versions of the EAX, EBX, ECX, EDX, EDI, ESI, ESP, and EBP registers defined in the x86 processor architecture, respectively). Additionally, in one embodiment, register file 22 may include additional registers addressed using a register extension (REX) prefix byte. Register file 22 may further include the RIP register, which may be a 64 bit version of the EIP register. Alternatively, execution core 14 may employ a form of register renaming in which any register within register file 22 may be mapped to an architected register. The number of registers in register file 22 may be implementation dependent for such an embodiment.

[0050] Data cache 16 is a high speed cache memory configured to store data. Data cache 16 may employ any suitable cache organization, including direct-mapped, set associative, and fully associative configurations. If a data fetch or update misses in data cache 16, data cache 16 may communicate with external interface unit 18 to fill the missing cache line into data cache 16. Additionally, if data cache 16 employs a writeback caching policy, updated cache lines which are being cast out of data cache 16 may be communicated to external interface unit 18 to be written back to memory. Data cache 16 may communicate with MMU 20 to receive physical address translations for virtual addresses presented to data cache 16.

[0051] External interface unit 18 communicates with portions of the system external to processor 10. External interface unit 18 may communicate cache lines for instruction cache 12 and data cache 16 as described above, and may communicate with MMU 20 as well. For example, external interface unit 18 may access the segment descriptor tables and/or paging tables on behalf of MMU 20.

[0052] It is noted that processor 10 may include an integrated level 2 (L2) cache, if desired. Furthermore, external interface unit 18 may be configured to communicate with a backside cache in addition to communicating with the system.

[0053] While the processor architecture described herein may be compatible with the x86 processor architecture for 16 and 32 bit modes, in one embodiment, other embodiments may employ any 16 and 32 bit modes. The other embodiments may or may not be compatible with the x86 processor architecture or any other processor architecture. It is further noted that, while a specific set of information is described herein as being used to generate the operating mode, any combination of indications and/or information from memory data structures such as segment descriptor tables and page tables may be used to generate the operating mode in various embodiments.

[0054] Turning now to FIG. 2, a block diagram of one embodiment of a code segment descriptor 40 for 32/64 mode is shown. Other embodiments are possible and contemplated. In the embodiment of FIG. 2, code segment descriptor 40 comprises 8 bytes with the most significant 4 bytes illustrated above the least significant 4 bytes. The most significant four bytes are stored at a numerically larger address than the least significant four bytes. The most significant bit of each group of four bytes is illustrated as bit 31 in FIG. 2 (and FIG. 3 below), and the least significant bit is illustrated as bit 0. Short vertical lines within the four bytes delimit each bit, and the long vertical lines delimit a bit but also delimit a field (both in FIG. 2 and in FIG. 3).

[0055] Unlike the 32 bit and 16 bit code segment descriptors illustrated in FIG. 3 below, code segment descriptor 40 does not include a base address or limit. Processor 10 employs a flat virtual address space for 32/64 mode (rather than the segmented linear address space employed in 32 bit and 16 bit modes). Accordingly, the portions of code segment descriptor 40 which would otherwise store the base address and limit are reserved in segment descriptor 40. It is noted that a virtual address provided through segmentation may also be referred to herein as a “linear address”. The term “virtual address” encompasses any address which is translated through a translation mechanism to a physical address actually used to address memory, including linear addresses and other virtual addresses generated in non-segmented architectures.

[0056] Segment descriptor 40 includes a D bit 42, an L bit 44 (set to one for a 32/64 mode code segment), an available bit (AVL) 46, a present (P) bit 48, a descriptor privilege level (DPL) 50, and a type field 52. D bit 42 and L bit 44 are used to determine the operating mode of processor 10, as illustrated in FIG. 4 below. AVL bit 46 is available for use by system software (e.g. the operating system). P bit 48 is used to indicate whether or not the segment is present in memory. If P bit 48 is set, the segment is present and code may be fetched from the segment. If P bit 48 is clear, the segment is not present and an exception is generated to load the segment into memory (e.g. from disk storage or through a network connection). The DPL indicates the privilege level of the segment. Processor 10 employs four privilege levels (encoded as 0 through 3 in the DPL field, with level 0 being the most privileged level). Certain instructions and processor resources (e.g. configuration and control registers) are only executable or accessible at the more privileged levels, and attempts to execute these instructions or access these resources at the lower privilege levels result in an exception. When information from code segment 40 is loaded into the CS segment register, the DPL becomes the current privilege level (CPL) of processor 10. Type field 52 encodes the type of segment. For code segments, the most significant bit two bits of type field 52 may be set (the most significant bit distinguishing a code or data segment from a system segment, and the second most significant bit distinguishing a code segment from a data segment), and the remaining bits may encode additional segment type information (e.g. execute only, execute and read, or execute and read only, conforming, and whether or not the code segment has been accessed).

[0057] It is noted that, while several indications in the code segment descriptor are described as bits, with set and clear values having defined meanings, other embodiments may employ the opposite encodings and may use multiple bits, as desired. Thus, for example, the D bit 42 and the L bit 44 may each be an example of an operating mode indication which may be one or more bits as desired, similar to the discussion of enable indications above.

[0058] Turning now to FIG. 3, a block diagram of one embodiment of a code segment descriptor 54 for 32 and 16 bit compatibility mode is shown. Other embodiments are possible and contemplated. As with the embodiment of FIG. 2, code segment descriptor 54 comprises 8 bytes with the most significant 4 bytes illustrated above the least significant 4 bytes.

[0059] Code segment descriptor 54 includes D bit 42, L bit 44, AVL bit 46, P bit 48, DPL 50, and type field 52 similar to the above description of code segment descriptor 40. Additionally, code segment descriptor 54 includes a base address field (reference numerals 56A, 56B, and 56C), a limit field (reference numerals 57A and 57B) and a G bit 58. The base address field stores a base address which is added to the logical fetch address (stored in the RIP register) to form the linear address of an instruction, which may then optionally be translated to a physical address through a paging translation mechanism. The limit field stores a segment limit which defines the size of the segment. Attempts to access a byte at a logical address greater than the segment limit are disallowed and cause an exception. G bit 58 determines the scaling of the segment limit field. If G bit 58 is set the limit is scaled to 4K byte pages (e.g. 12 least significant zeros are appended to the limit in the limit field). If G bit 58 is clear, the limit is used as is.

[0060] It is noted that code segment descriptors for 32 and 16 bit modes when long mode is not active may be similar to code segment descriptor 54, except the L bit is reserved and defined to be zero. It is further noted that, in 32 and 16 bit modes (both compatibility mode with the LMA bit set and modes with the IMA bit clear) according to one embodiment, data segments are used as well. Data segment descriptors may be similar to code segment descriptor 54, except that the D bit 42 is defined to indicate the upper bound of the segment or to define the default stack size (for stack segments).

[0061] Turning next to FIG. 4, a table 70 is shown illustrating the states of the LMA bit, the L bit in the code segment descriptor, and the D bit in the code segment descriptor and the corresponding operating mode of processor 10 according to one embodiment of processor 10. Other embodiments are possible and contemplated. As table 70 illustrates, if the LMA bit is clear, then the L bit is reserved (and defined to be zero). However, processor 10 may treat the L bit as a don't care if the LMA bit is clear. Thus, the x86 compatible 16 bit and 32 bit modes may be provided by processor 10 if the LMA bit is clear. If the LMA bit is set and the L bit in the code segment is clear, then a compatibility operating mode is established by processor 10 and the D bit selects 16 bit or 32 bit mode. If the LMA bit and the L bit are set and the D bit is clear, 32/64 mode is selected for processor 10. Finally, the mode which would be selected if the LMA, L and D bits are all set is reserved.

[0062] As mentioned above, the 32/64 operating mode includes a default address size in excess of 32 bits (implementation dependent but up to 64 bits) and a default operand size of 32 bits. The default operand size of 32 bits may be overridden to 64 bits via a particular instruction's encoding. The default operand size of 32 bits is selected to minimize average instruction length (since overriding to 64 bits involves including an instruction prefix in the instruction encoding which may increase the instruction length) for programs in which 32 bits are sufficient for many of the data manipulations performed by the program. For such programs (which may be a substantial number of the programs currently in existence), moving to a 64 bit operand size may actually reduce the execution performance achieved by the program (i.e. increased execution time). In part, this reduction may be attributable to the doubling in size in memory of the data structures used by the program when 64 bit values are stored. If 32 bits is sufficient, these data structures would store 32 bit values, Thus, the number of bytes accessed when the data structure is accessed increases if 64 bit values are used where 32 bit values would be sufficient, and the increased memory bandwidth (and increased cache space occupied by each value) may cause increased execution time. Accordingly, 32 bits is selected as the default operand size and the default may be overridden via the encoding of a particular instruction.

[0063] Multiple Entry Points for System Call Instruction

[0064] The processor described above may be used in a system in which an operating system operates in 32/64 mode and various application programs operate in either 32/64 mode or one of the compatibility modes. Thus, the operands for the operating system service may vary in form depending on the operating mode of the calling application program (the “caller”). For example, FIG. 5 illustrates operand passing both in a register and on the stack for a 32 bit caller and a 64 bit caller. Other embodiments are possible and contemplated. A register 80 and a stack 82 are illustrated for a 32 bit caller, and a register 84 and a stack 86 are illustrated for a 64 bit caller.

[0065] Register 80 is illustrated as a 64 bit register, since processor 10 may also support 64 bit operands. The operand passed in register 80 is stored in the least significant 32 bits of the register. Since, when processor 10 is operating in 32 bit mode, 32 bit results are generated for instructions, the most significant 32 bits of register 80 may be undefined. The operand may be sign extended or zero extended to the full register width by the called routine. On the other hand, if the caller is 64 bit (e.g. operating in 32/64 mode), the operand may already be stored as a 64 bit value. Sign extending or zero extending the least significant 32 bits would be incorrect for the 64 bit caller.

[0066] Stacks 82 and 86 are illustrated in FIG. 5 as having 32 bit entries, merely for comparison between the stacks. Stack 82 may be pushed and popped in 32 bit portions (since the caller is operating in 32 bit mode), while stack 86 may be pushed and popped in 64 bit portions (since the caller is operating in 32/64 mode). The top of each stack is illustrated by an arrow labeled “TOS” in FIG. 5. Accordingly, in stack 82, operand A is stored at a zero offset from the top of stack and is 32 bits (4 bytes), operand B is stored at an offset of 4 bytes from the top of stack and is 4 bytes, operand C is stored at an offset of 8 bytes from the top of stack and is four bytes, etc. On the other hand, in stack 86, operand A is stored at an offset of zero from the top of stack but is 64 bits (8 bytes) in size, and thus operand B is stored at an offset of 8 bytes from the top of stack. As FIG. 5 illustrates, locating operands and determining how many bytes are included in the operand may depend on the caller's operating mode. A similar problem may exist for operands stored in memory locations which are not operated as a stack.

[0067] When long mode is active, processor 10 selects the target address of the system call instruction from one of multiple source address registers dependent on the operating mode of the caller. In this manner, the entry point into the operating system (i.e. the first code to execute in response to the system call instruction, stored at the target address selected in response to executing the system call instruction) may be different for different operating modes. Thus, different code may be executed based on the caller's operating mode. The operands may be translated from the form provided by the caller (using code at the various entry points) to a consistent form used by the operating system code. Then the entry point code may branch to the called operating system routine. The same called operating system routine may execute regardless of the operating mode of the caller. Furthermore, the code which translates the operands may be stored at the various entry points, and thus may effectively be shared by each of the callable operating system routines. It is noted that one of the operands provided by the caller may identify the called operating system routine. It is further noted that the labels “operand A”, “operand B”, etc. are not intended to imply any order of the operands (such as the order the operands may have been pushed, the order that they may be processed by the called routine, etc.).

[0068]FIG. 5A is an example of the use of different entry point code as described above. Illustrated in FIG. 5 is a 32 bit caller code sequence 182, a 64 bit caller code sequence 184, a 32 bit entry point code sequence 186, a 64 bit entry point code sequence 188, and three operating system service code sequences 190, 192, and 194. The 32 bit caller code sequence 182 includes a system call instruction which, when executed, branches to the 32 bit entry point code sequence 186 due to selection of the target address based on the caller's operating mode. The 32 bit entry point code sequence 186 may translate the operands to the form expected by the called operating system service, and then may branch to that called operating system service code sequence 190, 192, or 194. Similarly, the 64 bit caller code sequence 184 includes a system call instruction which, when executed, branches to the 64 bit entry point code sequence 188 due to selection of the target address based on the caller's operating mode. The 64 bit entry point code sequence 188 may translate the operands to the form expected by the called operating system service, and then may branch to that called operating system service code sequence 190, 192, or 194. It is noted that, in some embodiments, the 64 bit operands may not require translation (the translation routine may be null). Not shown in FIG. 5A is the return, via a system return instruction with an appropriate operand size, to the calling code sequence 182 or 184. It is noted that, while FIG. 5A shows separate entry point code sequences 186-188 with shared operating system service code sequences 190-194, other embodiments may employ separate sets of operating system service code sequences for different calling operating modes, or other alternative methods, as desired.

[0069] It is noted that, while 32 bit and 64 bit sizes are shown in the example of FIG. 5, any two (or more) sizes may be used. It is further noted that, while processor 10 selects from multiple target addresses in the present embodiment when long mode is active, other embodiments may select from multiple target addresses in any operating mode, as desired.

[0070] Turning now to FIG. 6, a block diagram illustrating one embodiment of MSRs 36A-36C is shown. Other embodiments are possible and contemplated. In the embodiment of FIG. 6, MSR 36A may also be referred to as the SYSCALL/SYSRET Target Address Register (STAR) and includes a system return segment selector (Sysret Selector) field 90, a system call segment selector (Syscall Selector) field 92, and a target address (target IP) field 94. MSR 36B may also be referred to as the Long Mode STAR (LSTAR) and includes a target address (target IP) field 96. Finally, MSR 36C may also be referred to as the Compatibility Mode STAR (CSTAR) and includes a target address (target IP) field 98.

[0071] If long mode is active, the target address selected during execution of the system call instruction is either the target address from MSR 36B or MSR 36C, dependent of the operating mode of the caller. More particularly, if the caller's operating mode is 32/64 mode, the target address is selected from MSR 36B. If the caller is operating in compatibility mode, the target address is selected from MSR 36C. If long mode is not active (and thus processor 10 is operating in legacy mode), the target address is selected from MSR 36A.

[0072] Additionally, in any operating mode, the code segment register and stack segment register may be changed. Particularly, the privilege level may be changed and operating mode information may be changed as set forth below in FIGS. 7 and 8. The segment selectors used in the code segment register and stack segment register may be the selectors in Syscall selector field 92 (during execution of the system call instruction) and in Sysret selector field 90 (during execution of the system return instruction), or the selectors may be derived from the selectors in fields 92 and 90.

[0073] It is noted that, while MSRs 36A-36C are model specific registers in this embodiment, other embodiments may implement these registers as special purpose registers, general purpose registers, configuration registers, or any other type of register. Still further, the target addresses may be stored in memory locations, if desired.

[0074] As used herein the term “target address” refers to an address which is branched to in response to a system call instruction. In other words, the program counter is set to the target address in response to executing the system call instruction. The program counter stores the address of the instruction being executed. Thus, the next program counter address after execution of the system call instruction is the target address. Therefore, the next instruction to be executed according to the program order of instructions is the instruction stored at the target address. Generally, processor 10 may, in some embodiments, employ speculative and/or out of order execution in which case some instructions may be speculatively executed concurrent with the system call instruction or between the system call instruction and the target instruction at the target address.

[0075] Turning now to FIG. 7, a flowchart is shown illustrating operation of one embodiment of execution core 14 during execution of a system call instruction. Other embodiments are possible and contemplated. While the blocks shown in FIG. 7 may be illustrated in a particular order for ease of understanding, any suitable order may be used. Execution core 14 may include a microcode routine which includes instructions performing the blocks of FIG. 7. Alternatively, combinatorial logic in execution core 14 may perform the block shown in FIG. 7 (and may perform blocks in parallel).

[0076] Execution core 14 copies the contents of STAR[47:32] (the Syscall selector field 92) into the selector field of the code segment (CS) register and copies the Syscall selector field 92 incremented by eight to the selector field of the stack segment (SS) register (block 100). Since, in the x86 architecture, segment descriptor table entries are 8 bytes, the stack segment selector indicates the next consecutive descriptor in the segment descriptor table to the code segment descriptor indicated by the code segment selector.

[0077] Execution core 14 determines if long mode is active in processor 10 (decision block 102). If long mode is not active, the processor 10 is operating in legacy mode in which processor 10 is compatible with the x86 processor architecture. Execution core 14 may copy the EIP (the 32 bit program counter address) of the instruction following the system call instruction to the ECX register (block 104), copy the contents of STAR[3 1:0] (the target address field 94) into the EIP register (block 106), set the segment descriptor portion of the CS segment register to a flat, 4 GB, read-only, 32 bit (CS.L=0, CS.D=1) legacy segment with the privilege level equal to zero (block 108), set the segment descriptor portion of the SS segment register to a flat, 4 GB, read/write and expand up, 32 bit segment (block 110), and may update the flags register (EFLAGS) to clear the IF and VM flags (block 112).

[0078] Accordingly, if long mode is not active, a privilege level change to privilege level 0 (most privileged) and a flat 32 bit code segment/stack segment occurs in response to the system call instruction. A “flat” segment is one in which the segment base address is set to zero and the segment limit is set to the maximum limit (4 GB), and thus the logical address is equal to the linear address. Storing the EIP of the instruction following the system call instruction in the ECX register provides a return address for the system return instruction.

[0079] If long mode is active (decision block 102), the RIP of the instruction following the system call instruction is copied into RCX (thus providing a 64 bit return address for the system return instruction—block 114), the flags register may be copied to R11 (one of the expanded registers provided for using the REX prefix byte mentioned above) to preserve the flags from the caller (block 116). Additionally, the flags may optionally be updated. In one embodiment, the operating system may specify a flags mask (e.g. in an MSR, a configuration register, a general register, etc.) which indicates which flags bits to clear and which to leave unmodified. Execution core may determine if the caller is operating in compatibility mode (decision block 118), and may select the target address from one of the LSTAR register (MSR 36B) or the CSTAR register (MSR 36C) dependent on whether or not the caller is operating in compatibility mode. If the caller is operating in compatibility mode, the target address is selected from CSTAR register (block 120). If the caller is operating in 32/64 mode, the target address from the LSTAR register is selected (block 122). The RIP register is updated with the selected target address. In other words, the next program counter address after the system call instruction is one of the addresses in the CSTAR or LSTAR register, dependent on the operating mode.

[0080] Finally, execution core 14 may set the CS descriptor information to indicate a 32/64 mode read-only segment (CS.L=1, CS.D=0) with a privilege level of zero (block 124). Since other segment registers are not used in 32/64 mode, the SS segment register may be unmodified.

[0081] Turning next to FIG. 8, a flowchart is shown illustrating operation of one embodiment of execution core 14 during execution of a system return instruction. Other embodiments are possible and contemplated. While the blocks shown in FIG. 8 may be illustrated in a particular order for ease of understanding, any suitable order may be used. Execution core 14 may include a microcode routine which includes instructions performing the blocks of FIG. 8. Alternatively, combinatorial logic in execution core 14 may perform the block shown in FIG. 8 (and may perform blocks in parallel).

[0082] Execution core 14 determines if 32/64 mode is active (decision block 130). If 32/64 mode is not active, execution core 14 copies the contents of STAR[63:48] (the Sysret selector field 90) into the selector portion of the CS segment register and the contents of STAR[63:48] incremented by eight into the selector portion of the SS segment register (block 132). Execution core 14 copies the contents of ECX into the EIP (block 134), thus returning to the instruction following the system call instruction which corresponds to the system return instruction. Execution core 14 sets the segment descriptor portion of the CS segment register to a flat, 4 GB, read-only, 32 bit (CS.LI-0, CS.D=1) legacy segment with the privilege level equal to three (block 136). Execution core 14 sets the segment descriptor portion of the SS segment register to a flat, 4 GB, read/write and expand up, 32 bit segment (block 138), and may update the flags register (EFLAGS) to set the IF flags (block 140).

[0083] On the other hand, if 32/64 mode is active, execution core 14 determines if the operand size is 64 bits (decision block 142). In the present embodiment, the operand size for the system return instruction determines the operating mode established via the CS segment register during execution of the system return instruction (and thus the operating mode of the code being returned to). The code at the entry points provided based on the different operating modes of the caller may pass the operating mode of the caller as an operand to the various operating system routines, and the routines may use this information to determine which operand size to use in the system return instruction. In one embodiment, the default operand size for the system return instruction is 32 bit and an operand size override prefix may be used to select the 64 bit operand size. Other embodiments may encode operand size in the instructions in other ways (e.g. inherent in the opcode, or in some other instruction field).

[0084] If the operand size is not 64 bit, execution core 14 may copy the contents of STAR[63:48] (the Sysret selector field 90) into the selector portion of the CS segment register (block 144). Additionally, execution core 14 may copy the contents of STAR[63:48] incremented by 8 into the selector portion of the SS segment register (block 146). Execution core 14 may set the descriptor portion of the CS segment register to a flat, 4 GB, read-only segment (CS.L0, CS.D=1) with a privilege level of 3 (block 148), and the descriptor portion of the SS segment register to a flat 4 GB read/write and expand up segment (CS.L0, CS.D=1) (block 150).

[0085] On the other hand, if the operand size is 64 bit, execution core 14 may load the contents of STAR[63:48] (the Sysret selector field 90) incremented by 16 into the selector portion of the CS segment register (block 152). Thus, the segment selector in the CS register may indicate the segment descriptor in a segment descriptor table entry two entries above the segment descriptor located by the Sysret selector field 90. In this manner, a different segment descriptor is indicated depending on the operating mode being established. Thus, segment descriptors matching the information stored into the descriptor portion of the CS segment registers may be placed in the corresponding segment descriptor table entries. Additionally, execution core 14 may set the descriptor portion of the segment register to a 32/64 mode read-only segment (CS.L32 1, CS.D=0) with a privilege level of three (block 154).

[0086] In either case, execution core 14 may copy the contents of RCX into RIP, thus selecting the previously saved return address as the next program counter address (block 156) and may copy R11 to the flags register (block 158), thus restoring the flags register to the precall state. Optionally, the flags may be modified after restoration from R11 (e.g. the VM bit may be cleared, various reserved bits may be cleared or set, etc.).

[0087] It is noted that the descriptor portions of the CS (and SS) segment registers are changed by execution core 14 in FIGS. 7 and 8 without reference to the segment descriptor table. Accordingly, the segment descriptors in the segment descriptor table entry corresponding to the segment selector in Syscall selector field 92, the Sysret selector field 90 and the next two consecutive segment descriptor table entries should be created to match the information that processor 10 inserts into the CS (and SS) segment registers. Particularly, a first segment descriptor in a descriptor table entry indicated by the segment selector stored in Sysret selector field 90 should be coded as a flat, 32 bit, 4 GB code segment with a DPL of three. A second segment descriptor, which is in the next consecutive segment descriptor table entry to the first segment descriptor, should be coded as a flat, 32 bit, 4 GB stack segment (expand up) with a DPL of three. A third segment descriptor, which is in the next consecutive segment descriptor table entry to the second segment descriptor, should be coded as a 32/64 mode code segment with a DPL of three. A fourth segment descriptor in a descriptor table entry indicated by the segment selector stored in Syscall selector field 92 should be coded as a flat, 32 bit, 4 GB code segment with a DPL of zero or as a 32/64 mode code segment with a DPL of three, dependent on the operating mode of the called code sequence. A fifth segment descriptor, which is in the next consecutive segment descriptor table entry to the fourth segment descriptor, should be coded as a flat, 32 bit, 4 GB stack segment (expand up) with a DPL of zero.

[0088] It is further noted that, in other embodiments, other operating modes may be defined than the set of operating modes defined herein. Any set of two or more operating modes may be defined, and the target address may be selected based on the active operating mode. Furthermore, embodiments in which segmentation is not employed are contemplated, and thus the blocks for modifying segment registers in FIGS. 7 and 8 may be omitted.

[0089] It is noted that, while a descriptor table entry size of 8 bytes is used in FIGS. 7 and 8, any size entry may be used. The descriptor table entry located by the selector fields in MSR 36A may be incremented by once and twice the size of the descriptor table entry where 8 and 16 were used above, respectively.

[0090] Turning now to FIG. 9, a block diagram illustrating one embodiment of a system call instruction 170 and one embodiment of a system return instruction 172 is shown. Other embodiments are possible and contemplated.

[0091] In the embodiment of FIG. 9, system call instruction 170 includes two bytes, the first of which is encoded as OF (in hexadecimal) and the second of which is encoded as 05 (in hexadecimal). As mentioned above, the target address selected during execution of the system call instruction is dependent on the operating mode.

[0092] In the embodiment of FIG. 9, system return instruction 172 includes two bytes and optionally a prefix byte 180. The other two bytes are encoded as OF and 07 (expressed in hexadecimal).

[0093] Prefix byte 180 may be the REX prefix byte mentioned above. Prefix byte 180 may be included in system return instruction 172 to override the default operand size of the instruction to 64 bits. More particularly, the embodiment illustrated in FIG. 9 encodes the most significant 4 bits of prefix byte 180 with a value of 4 in hexadecimal to identify the prefix byte. The least significant 4 bits include a bit labeled “W” which, if set, indicates that the operand size of the instruction is overridden to 64 bits. Furthermore, in the embodiment shown, the X, Y, and Z bits may be used to extend the register addresses that may appear in various instructions, thus providing for 8 additional registers in an embodiment compatible with the x86 architecture. In one example, the X bit may be used to provide the most significant register address bit for the “reg” field of the addressing mode (Mod R/M) byte of instructions. The Y bit may be used to provide the most significant register address bit for the index field of the scale-index-base (SIB) byte of instructions. The Z field may be used to provide the most significant register address bit for r/m field of the Mod R/M byte, the base field of the SIB byte, and the opcode register address field included in some instructions.

[0094] While the illustrated embodiment may be a variable byte length instruction set (e.g. compatible with the x86 architecture), other embodiments are contemplated for other variable byte length instruction sets and fixed length instructions sets. While a prefix byte is used to change operand size in the illustrated embodiment, other embodiments may use other methods (e.g. different opcode encodings for different operand sizes, coding the operand size in another instruction field, etc.).

[0095] Software Embodiments

[0096] While the above description may generally have described a processor which may directly support, in hardware, the processor architecture having the features described above, it is contemplated that other processor embodiments may not directly implement the processor architecture. Instead, such embodiments may directly implement a different processor architecture (referred to below as a native processor architecture, which may define a native instruction set including native instructions). Any native processor architecture may be used. For example, the MIPS, Power PC, Alpha, Sparc, ARM, etc. architectures may be used. The processor architecture may be implemented in software executing on the native processor architecture in a variety of fashions, using any native processor architecture such as, for example, the Crusoe products of Transmeta Corporation.

[0097] Generally, a processor embodiment implementing a native processor architecture different than the processor architecture described above (referred to below as the non-native processor architecture) may support the non-native processor architecture in a variety of fashions. For example, such a processor embodiment may execute interpreter software which reads each non-native instruction in a non-native code sequence as data, and executes various software routines which emulate the defined operation of the non-native instruction as defined in the non-native processor architecture. Alternatively, translator software may be executed. The translator software may translate the non-native instructions in the code sequence to an equivalent set of native instructions defined by the native instruction set architecture. The native code sequence may be stored in memory, and may be executed instead of the corresponding non-native code sequence. In yet another alternative, a mixture of interpretation and translation may be used. For example, the code sequence may be interpreted, but the interpreter may also generate statistics about which parts of the code sequence are being most frequently executed. The most frequently executed portions may then be translated to native code sequences.

[0098] In any of the above methods, the architected state defined by the non-native processor architecture may be maintained by the combination of the processor and the software (interpreter or translator) in a variety of fashions. For example, the non-native architected state may be mapped to memory locations in a memory addressable by the processor, to general registers defined by the native processor architecture (by software convention, either in the interpreter or in the translator), or the processor may directly support the non-native architected state by defining registers or other storage hardware within the processor that corresponds to the non-native architected state. The non-native architected state may be stored using any combination of the above methods, as desired.

[0099] Generally, the architected state includes any state defined to exist by the architecture. For example, in the above described embodiment, the non-native architected state may include general registers (e.g. RAX, RBX, etc.), segment registers, control registers, other registers such as the model specific registers (MSRs), etc. Additionally, the architected state may include data structures defined for the operating system to create, such as the descriptor tables, page tables, task state segments, etc.

[0100] Turning to FIG. 10, a flowchart illustrating an exemplary interpreter which may be used to interpret non-native instructions is shown. Other embodiments are possible and contemplated. While the blocks shown are illustrated in a particular order for ease of understanding, any suitable order may be used. Furthermore, blocks may be performed in parallel, as desired.

[0101] The blocks shown in FIG. 10 illustrate the emulation of one non-native instruction. Generally, the interpreter may execute the blocks shown in FIG. 10 for each non-native instruction to be executed according to the non-native code sequence to be executed.

[0102] The interpreter may determine the operating mode for the non-native instruction (block 1000). As described above, the operating mode may be determined from the IMA bit in control register 26 and the L bit and D bit from the code segment descriptor indicated by the CS segment register. The operating mode may be determined anew from the LMA, L bit, and D bit for each non-native instruction, or the resulting operating mode may be stored in a temporary register for access by the interpreter for each non-native instruction. If the resulting operating mode is stored, the interpreter may update the stored operating mode if an instruction modifies the CS segment register or interrupt or exception handling causes the operating mode to change. As mentioned above, the CS segment register and the control register(s) (which are part of the non-native architected state) may actually be memory locations, general registers, or special purpose registers, or any combination thereof.

[0103] The interpreter may read the current non-native instruction from memory, and may analyze the non-native instruction to determine the operations to be taken to emulate the non-native instruction (block 1002). The interpreter may read the non-native instruction one byte at a time, or may read a suitable set of consecutive bytes and process the bytes. For example, a native processor architecture in which operands are 32 bit may read 32 bits (4 bytes) of the non-native instruction at a time, and then may process the four bytes before reading any additional bytes.

[0104] Generally, the interpreter software may decode the non-native instruction in a manner analogous to processor 10 decoding the instruction in hardware. Thus, for the illustrated non-native processor architecture, which is compatible with the x86 processor architecture, the analyzing of the non-native instruction includes analyzing any prefix bytes which may precede the opcode byte, analyzing the opcode byte, analyzing the addressing mode (Mod R/M) byte (if present), and analyzing the scale-index-base (SIB) byte (if present). Prefix bytes may override the operating mode, and may also include register specifier bits (e.g. the REX prefix byte). The opcode byte specifies the operation to be performed, and in some cases may include a register specifier or may implicitly specify an operand (e.g. the stack or the stack pointer). The Mod R/M byte specifies operands (including any displacement operands which may follow the Mod RIM byte or the SIB byte, if the SIB byte is present) and may include register specifiers. Finally, the SIB byte may include register specifiers. From the information gained from analyzing the non-native instruction, the interpreter has the information to emulate the non-native instruction (including operating mode for the non-native instruction, which specifies the operand size and address size of the non-native instruction, operands, the operation to be performed, etc.).

[0105] If the non-native instruction includes a memory operand (decision block 1004), the interpreter may calculate the effective address of the instruction (block 1006). If the non-native instruction has a memory operand, some of the operands identified in block 1002 may be address operands used to generate the effective address. Thus, the interpreter may read the address operands from the non-native architected state and may add them to generate an effective address. The size of the effective address may be determined by the address size for the instruction, as determined at blocks 1000 and 1002. It is noted that the native processor architecture may support an address size which is less than the address size supported by the non-native processor architecture. For example, in one exemplary embodiment described above, the virtual address size may be 48 bits in 32/64 mode. The native processor may, for example, support a virtual address size of 32 bits. In such an embodiment, block 1006 may represent a series of calculations in which the least significant bits (e.g. 32 bits) of the virtual address may be calculated, and any carry from the least significant bits may be carried into a calculation of the most significant bits of the virtual address.

[0106] The interpreter may then perform the operation specified by the non-native instruction (block 1008). If the non-native instruction includes a memory operand as a source operand, the interpreter may read the memory operand from the effective address calculated at block 1006. Other operands may be read from the non-native architected state. The operation may include an arithmetic operation, a logical operation, a shift, a move to another storage location, etc. The native processor architecture may support an operand size smaller than the operand size of the instruction. In such cases, performing the operation may include multiple calculations on portions of the operand to calculate the result.

[0107] The interpreter determines if the non-native instruction resulted in an exception (decision block 1010). Generally, exceptions may occur throughout the execution of the operations specified by the non-native instruction. For example, accessing a source memory operand may result in a page fault before any of the actual instruction operation is performed. During the operations, various architecturally-defined exceptions may also occur. The interpreter may interrupt processing of the non-native instruction upon detecting an exception, and may branch to exception handler instructions (block 1012). The exception handler may be native code or non-native code or a combination thereof, as desired. If the non-native processor architecture specifies the update of any architected state when an exception is taken (e.g. various control registers may store the address of the exception causing instruction, the exception reason, etc.), the interpreter may update the non-native architected state as defined.

[0108] It is noted that the interpreter software is executing on the native processor, and thus is subject to experiencing exceptions as defined in the native processor architecture. These exceptions may generally be different from the exceptions detected by the interpreter software, which are exceptions experienced by the non-native code being interpreted according to the non-native processor architecture.

[0109] If no exception occurs during emulation of the non-native instruction, the interpreter may update the non-native architected state according to the definition of the non-native instruction (block 1014). Finally, the interpreter may calculate the next non-native instruction fetch address to fetch the next instruction (block 1016). The next fetch address may be sequential to the current non-native instruction, or may be a different address (e.g. if the current non-native instruction is a taken branch, the next fetch address may be the target address of the branch instruction).

[0110] It is noted that the interpreter may operate in protected mode, using virtual addresses. In other words, the effective address calculated at block 1006 may be a virtual address which is translated by the translation mechanism specified by the non-native processor architecture to a physical address. The processor may include a translation lookaside buffer (TLB) used to cache translations. The processor may either support reload of the TLB from the non-native translation tables (page tables), or may take an exception on a TLB miss to allow software reload of the TLB.

[0111] Generally, the interpreter may perform the flowcharts of FIGS. 7 and 8 at any suitable point in the processing of instructions, e.g. blocks 1000, 1002, 1006, 1008, 1014, and/or 1016, depending on the instruction.

[0112] Turning to FIG. 11, a flowchart illustrating an exemplary translator which may be used to translate non-native instructions in the non-native processor architecture to native instructions in the native processor architecture. Other embodiments are possible and contemplated. While the blocks shown are illustrated in a particular order for ease of understanding, any suitable order may be used. Furthermore, blocks may be performed in parallel, as desired.

[0113] The blocks shown in FIG. 11 illustrate the translation of one non-native code sequence responsive to a fetch address for the first instruction in the non-native code sequence. The code translator may translate any number of non-native instructions to produce a translated code sequence having native instructions. For example, the translator may translate from the initial non-native instruction to a basic block boundary (i.e. a branch instruction). Alternatively, the translator may speculatively translate two or more basic blocks or may translate up to a maximum number of non-native or resulting native instructions, if desired.

[0114] Generally, the translator may maintain a translation cache which stores translated code sequences previously produced by the translator. The translation cache may identify translated code sequences by the fetch address of the first non-native instruction in the corresponding non-native code sequences. Thus, the translator may determine if a translated code sequence corresponding to the fetch address is stored in the translation cache (decision block 1030). If there is a translated code sequence in the translation cache, the translator may cause the processor to branch to that translated code sequence (block 1032). On the other hand, if there is no translated code sequence, the translator may translate one or more non-native instructions from the non-native code sequence into native instructions in a translated code sequence (block 1034).

[0115] Generally, the translator may translate each non-native instruction into one or more native instructions which, when executed, may perform the same operation on the non-native architected state that the non-native instruction would have performed. The translator may generally perform the same decoding of instructions as is performed by the interpreter (block 1002 in FIG. 10) to determine what operations may need to be performed. For example, if the native processor architecture is a load/store architecture in which memory operands are accessed using explicit load/store instructions and other instruction use only register operands, load and store instructions may be used to access the memory operands and other instructions may be used to perform the explicit operation of a non-native instruction having a memory operand. The translated instructions may make use of temporary registers to hold intermediate values corresponding to the execution of the non-native instruction. Additionally, the translated instructions may access the non-native architected state to retrieve operands and may update the non-native architected state with the final results of the non-native instruction. Generally, the native instructions corresponding to the non-native instruction may perform all of the operations defined for the instruction (e.g. blocks 1006, 1008, 1010, 1014, and 1016 in FIG. 10).

[0116] Once the translator has determined to terminate translation and save the translated sequence for execution, the translator may optionally optimize the translated code sequence (block 1036). The optimizations may include reordering the translated instructions for quicker execution, eliminating redundancies (e.g. redundant memory references, which may occur if multiple non-native instructions in the source code sequence accessed the same memory location), etc. Any suitable set of optimizations may be used. The resulting translated code sequence may then be stored into the translation cache. Additionally, the processor may branch to the translated code sequence and execute the sequence (block 1032).

[0117] It is noted that, while the above description may refer to accessing and/or updating non-native architected state, including various registers, the non-native architected state may be stored in any suitable fashion. For example, architected registers may actually be stored in memory locations, as highlighted above. The mapping of architected registers from the non-native processor architecture to memory locations may be used in either of the interpreter or the translator embodiments, or combinations thereof, to locate the non-architected state used during execution of the non-native instruction or affected by the execution of the non-native instruction. Thus, instructions which access the non-native architected state may perform memory reads/writes or register reads/writes, as the case may be.

[0118] Turning next to FIG. 12, a block diagram illustrating one exemplary mapping of non-native architected state to either memory locations in a memory 1040 or to processor resources in a native processor 1042. Native processor 1042 includes a register file 1044 including the architected general registers of the native processor architecture. Any number of registers may be provided.

[0119] In the embodiment of FIG. 12, all of the non-native architected state is mapped to memory 1040. For example, descriptor tables 1046 (which may include a global descriptor table, a local descriptor table, and an interrupt descriptor table), page tables 1048 (which store virtual to physical address translations), task state segments 1050, general registers 1052, segment registers 1054, control registers 1056, and other registers 1058 may represent non-native architected state.

[0120] Thus, in the embodiment of FIG. 12, to access any non-native architected state, a memory access may be performed. For example, if a non-native instruction has one of the general registers as an operand, the interpreter or translated native instruction performs a memory access to the memory location mapped to that general register to access or update that general register. The registers in register file 1044 may be used by the interpreter or translator as temporary registers to hold intermediate results or for other local interpreter/translator state.

[0121] General registers 1052 may include integer general registers (e.g. RAX, RBX, etc. as described above), the additional integer general registers defined by the REX prefix byte, floating point registers, Streaming Single Instruction, Multiple Data (SIMD) Extension (SSE) registers, and the additional SSE registers defined by the REX prefix byte.

[0122] Segment registers 1054 may include storage locations corresponding to the segment registers 24 shown in FIG. 1 above.

[0123] Control registers 1056 may include storage locations corresponding to various control registers defined in the non-native processor architecture. For example, control registers storing the LMA, LME, PG and PE bits, as well as the LDTR and GDTR registers and the CR3 register (which stores the base address of the page tables 1048) are shown. Other control registers may be included as well.

[0124] Other registers 1058 includes any remaining architected registers. For example, the EFLAGS register (which stores condition code information), the instruction pointer (RIP) register (which stores the address of the instruction to be executed), and the model specific registers (MSRs) may be included in other registers 1058.

[0125] While the example of FIG. 12 maps all of the non-native architected state to memory 1040, other embodiments may implement other mappings. In FIG. 13, for example, some of the general registers in register file 1044 are mapped to the general registers 1052. Accordingly, if a non-native instruction has a general register as an operand, the interpreter accesses the corresponding register in register file 1044. Similarly, the translator generates a translated instruction having the corresponding register in register file 1044 as an operand. Other architected state may still be accessed via memory operations in the embodiment of FIG. 13. Other registers in register file 1044 which are not assigned to non-native architected state may again be used as temporary registers for interpreter or translator use, as described above.

[0126] While the embodiment of FIG. 13 illustrates mapping the general registers 1052 to registers in register file 1044, any other non-native architected state may be mapped to registers in register file 1044. For example, any of segment registers 1054, control registers 1056, or other registers 1058 (or portions of any of these registers) may be mapped to register file 1044, as desired.

[0127]FIG. 14 illustrates another example in which the general registers 1052 and the EFLAGS and RIP registers are mapped to registers in register file 1044. Additionally, in the example of FIG. 14, the segment registers 1054 are implemented in hardware in processor 1042. More specifically, processor 1042 may not only implement storage for segment registers 1054, but may also include logic to generate the operating mode for instructions based on the information in the segment registers. Furthermore, for compatibility modes, the logic may include limit checks and attribute checks to ensure that accesses to the segment attempted by the non-native instructions (or the non-native instructions in the interpreter or the translated code sequence which correspond to the non-native instructions) are permitted.

[0128] Similarly, other embodiments may implement various control registers 1056 or other registers 1058 in hardware, including corresponding logic to act on the contents of the registers as defined in the non-native architecture. Generally, various embodiments of processor 1042 may implement any non-native architected state in hardware. Certain architected state may generally be implemented in memory since the non-native processor architecture defines the state to be in memory (e.g. descriptor tables 1046, pages tables 1048, and task state segments 1050). Such memory-based architected state may be cached in caches within processor 1042 (e.g. TLBs for page table information, hidden segment register portions for segment descriptor information, etc.).

[0129] As the above discussion illustrates, the non-native architected state may be stored in any suitable storage location. Generally, a storage location is a location capable of storing a value. Suitable storage locations may include, in various embodiments, a memory location, a general register mapped to the non-native architected state, or a special purpose register (which may include additional hardware to interpret the contents of the register), depending upon the embodiment. Additionally, suitable storage locations could include a scratch pad RAM (such as a portion of a cache predetermined to be used as scratch pad RAM).

[0130]FIG. 15 is a block diagram of one embodiment of a carrier medium 1090. Other embodiments are possible and contemplated. In the embodiment of FIG. 15, carrier medium 1090 stores an interpreter program 1092 and a translator program 1094.

[0131] Generally speaking, a carrier medium may include storage media such as magnetic or optical media, e.g., disk or CD-ROM, volatile or non-volatile memory media such as RAM (e.g. SDRAM, RDRAM, SRAM, etc.), ROM, etc., as well as transmission media or signals such as electrical, electromagnetic, or digital signals, conveyed via a communication medium such as a network and/or a wireless link. Carrier medium 1090 may thus be coupled to a computer system including processor 1042, may be part of a computer system including processor 1042, or may be a communication medium on which the computer system is capable of communicating. Computer systems including processor 1042 may be of any construction. For example, computer systems similar to those shown in FIGS. 16 and 17 may be suitable.

[0132] Interpreter program 1090 may operate according to the flowchart of FIG. 10. Translator program 1094 may operate according to the flowchart of FIG. 11. Generally, interpreter program 1092 and translator program 1094 may each comprise code sequences including native instructions.

[0133] Computer Systems

[0134] Turning now to FIG. 16, a block diagram of one embodiment of a computer system 200 including processor 10 coupled to a variety of system components through a a bus bridge 202 is shown. Other embodiments are possible and contemplated. In the depicted system, a main memory 204 is coupled to bus bridge 202 through a memory bus 206, and a graphics controller 208 is coupled to bus bridge 202 through an AGP bus 210. Finally, a plurality of PCI devices 212A-212B are coupled to bus bridge 202 through a PCI bus 214. A secondary bus bridge 216 may further be provided to accommodate an electrical interface to one or more EISA or ISA devices 218 through an EISA/ISA bus 220. Processor 10 is coupled to bus bridge 202 through a CPU bus 224 and to an optional L2 cache 228. Together, CPU bus 224 and the interface to L2 cache 228 may comprise an external interface to which external interface unit 18 may couple.

[0135] Bus bridge 202 provides an interface between processor 10, main memory 204, graphics controller 208, and devices attached to PCI bus 214. When an operation is received from one of the devices connected to bus bridge 202, bus bridge 202 identifies the target of the operation (e.g. a particular device or, in the case of PCI bus 214, that the target is on PCI bus 214). Bus bridge 202 routes the operation to the targeted device. Bus bridge 202 generally translates an operation from the protocol used by the source device or bus to the protocol used by the target device or bus.

[0136] In addition to providing an interface to an ISA/EISA bus for PCI bus 214, secondary bus bridge 216 may further incorporate additional functionality, as desired. An input/output controller (not shown), either external from or integrated with secondary bus bridge 216, may also be included within computer system 200 to provide operational support for a keyboard and mouse 222 and for various serial and parallel ports, as desired. An external cache unit (not shown) may further be coupled to CPU bus 224 between processor 10 and bus bridge 202 in other embodiments. Alternatively, the external cache may be coupled to bus bridge 202 and cache control logic for the external cache may be integrated into bus bridge 202. L2 cache 228 is further shown in a backside configuration to processor 10. It is noted that L2 cache 228 may be separate from processor 10, integrated into a cartridge (e.g. slot 1 or slot A) with processor 10, or even integrated onto a semiconductor substrate with processor 10.

[0137] Main memory 204 is a memory in which application programs are stored and from which processor 10 primarily executes. A suitable main memory 204 comprises DRAM (Dynamic Random Access Memory). For example, a plurality of banks of SDRAM (Synchronous DRAM) or Rambus DRAM (RDRAM) may be suitable.

[0138] PCI devices 212A-212B are illustrative of a variety of peripheral devices. The peripheral devices may include devices for communicating with another computer system to which the devices may be coupled (e.g. network interface cards, modems, etc.). Additionally, peripheral devices may include other devices, such as, for example, video accelerators, audio cards, hard or floppy disk drives or drive controllers, SCSI (Small Computer Systems Interface) adapters and telephony cards. Similarly, ISA device 218 is illustrative of various types of peripheral devices, such as a modem, a sound card, and a variety of data acquisition cards such as GPIB or field bus interface cards.

[0139] Graphics controller 208 is provided to control the rendering of text and images on a display 226. Graphics controller 208 may embody a typical graphics accelerator generally known in the art to render three-dimensional data structures which can be effectively shifted into and from main memory 204. Graphics controller 208 may therefore be a master of AGP bus 210 in that it can request and receive access to a target interface within bus bridge 202 to thereby obtain access to main memory 204. A dedicated graphics bus accommodates rapid retrieval of data from main memory 204. For certain operations, graphics controller 208 may further be configured to generate PCI protocol transactions on AGP bus 210. The AGP interface of bus bridge 202 may thus include functionality to support both AGP protocol transactions as well as PCI protocol target and initiator transactions. Display 226 is any electronic display upon which an image or text can be presented. A suitable display 226 includes a cathode ray tube (“CRT”), a liquid crystal display (“LCD”), etc.

[0140] It is noted that, while the AGP, PCI, and ISA or EISA buses have been used as examples in the above description, any bus architectures may be substituted as desired. It is further noted that computer system 200 may be a multiprocessing computer system including additional processors (e.g. processor 10a shown as an optional component of computer system 200). Processor 10 a may be similar to processor 10. More particularly, processor 10 a may be an identical copy of processor 10. Processor 10 a may be connected to bus bridge 202 via an independent bus (as shown in FIG. 16) or may share CPU bus 224 with processor 10. Furthermore, processor 10 a may be coupled to an optional L2 cache 228 a similar to L2 cache 228.

[0141] Turning now to FIG. 17, another embodiment of a computer system 300 is shown. Other embodiments are possible and contemplated. In the embodiment of FIG. 17, computer system 300 includes several processing nodes 312A, 312B, 312C, and 312D. Each processing node is coupled to a respective memory 314A-314D via a memory controller 316A-316D included within each respective processing node 312A-312D. Additionally, processing nodes 312A-312D include interface logic used to communicate between the processing nodes 312A-312D. For example, processing node 312A includes interface logic 318A for communicating with processing node 312B; interface logic 318B for communicating with processing node 312C, and a third interface logic 318C for communicating with yet another processing node (not shown). Similarly, processing node 312B includes interface logic 318D, 318E, and 318F; processing node 312C includes interface logic 318G, 318H, and 3181; and processing node 312D includes interface logic 318J, 318K, and 318L. Processing node 312D is coupled to communicate with a plurality of input/output devices (e.g. devices 320A-320B in a daisy chain configuration) via interface logic 318L. Other processing nodes may communicate with other I/O devices in a similar fashion.

[0142] Processing nodes 312A-312D implement a packet-based link for inter-processing node communication. In the present embodiment, the link is implemented as sets of unidirectional lines (e.g. lines 324A are used to transmit packets from processing node 312A to processing node 312B and lines 324B are used to transmit packets from processing node 312B to processing node 312A). Other sets of lines 324C-324H are used to transmit packets between other processing nodes as illustrated in FIG. 17. Generally, each set of lines 324 may include one or more data lines, one or more clock lines corresponding to the data lines, and one or more control lines indicating the type of packet being conveyed. The link may be operated in a cache coherent fashion for communication between processing nodes or in a noncoherent fashion for communication between a processing node and an I/O device (or a bus bridge to an I/O bus of conventional construction such as the PCI bus or ISA bus). Furthermore, the link may be operated in a non-coherent fashion using a daisy-chain structure between I/O devices as shown. It is noted that a packet to be transmitted from one processing node to another may pass through one or more intermediate nodes. For example, a packet transmitted by processing node 312A to processing node 312D may pass through either processing node 312B or processing node 312C as shown in FIG. 17. Any suitable routing algorithm may be used. Other embodiments of computer system 300 may include more or fewer processing nodes then the embodiment shown in FIG. 17.

[0143] Generally, the packets may be transmitted as one or more bit times on the lines 324 between nodes. A bit time may be the rising or falling edge of the clock signal on the corresponding clock lines. The packets may include command packets for initiating transactions, probe packets for maintaining cache coherency, and response packets from responding to probes and commands.

[0144] Processing nodes 312A-312D, in addition to a memory controller and interface logic, may include one or more processors. Broadly speaking, a processing node comprises at least one processor and may optionally include a memory controller for communicating with a memory and other logic as desired. More particularly, each processing node 312A-312D may comprise one or more copies of processor 10. External interface unit 18 may includes the interface logic 318 within the node, as well as the memory controller 316.

[0145] Memories 314A-314D may comprise any suitable memory devices. For example, a memory 314A-314D may comprise one or more RAMBUS DRAMs (RDRAMs), synchronous DRAMs (SDRAMs), static RAM, etc. The address space of computer system 300 is divided among memories 314A-314D. Each processing node 312A-312D may include a memory map used to determine which addresses are mapped to which memories 314A-314D, and hence to which processing node 312A-312D a memory request for a particular address should be routed. In one embodiment, the coherency point for an address within computer system 300 is the memory controller 316A-316D coupled to the memory storing bytes corresponding to the address. In other words, the memory controller 316A-316D is responsible for ensuring that each memory access to the corresponding memory 314A-314D occurs in a cache coherent fashion. Memory controllers 316A-316D may comprise control circuitry for interfacing to memories 314A-314D. Additionally, memory controllers 316A-316D may include request queues for queuing memory requests.

[0146] Generally, interface logic 318A-318L may comprise a variety of buffers for receiving packets from the link and for buffering packets to be transmitted upon the link. Computer system 300 may employ any suitable flow control mechanism for transmitting packets. For example, in one embodiment, each interface logic 318 stores a count of the number of each type of buffer within the receiver at the other end of the link to which that interface logic is connected. The interface logic does not transmit a packet unless the receiving interface logic has a free buffer to store the packet. As a receiving buffer is freed by routing a packet onward, the receiving interface logic transmits a message to the sending interface logic to indicate that the buffer has been freed. Such a mechanism may be referred to as a “coupon-based” system. I/O devices 320A-320B may be any suitable I/O devices. For example, I/O devices 320A-320B may include devices for communicate with another computer system to which the devices may be coupled (e.g. network interface cards or modems). Furthermore, I/O devices 320A-320B may include video accelerators, audio cards, hard or floppy disk drives or drive controllers, SCSI (Small Computer Systems Interface) adapters and telephony cards, sound cards, and a variety of data acquisition cards such as GPIB or field bus interface cards. It is noted that the term “I/O device” and the term “peripheral device” are intended to be synonymous herein.

[0147] Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications. 

1. A processor comprising: a first register configured to store a first target address; a second register configured to store a second target address; a third register, and an execution core coupled to the first register, the second register, and the third register, wherein the execution core is configured, responsive to a first instruction, to: (i) select the first target address from the first register as a next program counter address if a first operating mode is active in the processor, and (ii) select the second target address from the second register as the next program counter address if a second operating mode is active in the processor, and wherein the execution core is configured to store a first address of a second instruction following the first instruction in the third register responsive to the first instruction.
 2. The processor as recited in claim 1 further comprising a fourth register coupled to the execution core, a code segment register coupled to the execution core, and a stack segment register coupled to the execution core, wherein the execution core is configured, responsive to the first instruction, to: (i) read a first segment selector from the fourth register, (ii) store the first segment selector in the code segment register, and (iii) generate a second segment selector from the first segment selector and store the second segment selector in the stack segment register.
 3. The processor as recited in claim 2 wherein the fourth register is further configured to store a third target address, and wherein the execution core is configured, responsive to the first instruction, to select the third target address as the next program counter address if a third operating mode is active in the processor.
 4. The processor as recited in claim 1 wherein the first operating mode includes a default address size greater than 32 bits.
 5. The processor as recited in claim 1 further comprising a segment register configured to store a segment selector and at least a first operating mode indication and a second operating mode indication from a segment descriptor indicated by the segment selector, and still further comprising a configuration register storing an indication, wherein the processor is configured to generate an operating mode in response to the first operating mode indication, the second operating mode indication, and the indication in the configuration register.
 6. (Cancelled)
 7. The processor as recited in claim 1 wherein the execution core, responsive to a third instruction, is configured to branch to the first address, the processor further comprising a fourth register configured to store a first segment selector, and wherein the execution core is configured, responsive to the third instruction, to generate a second segment selector from the first segment selector responsive to an operand size of the third instruction.
 8. The processor as recited in claim 7 wherein the operand size of the third instruction is responsive to a prefix byte of the third instruction.
 9. The processor as recited in claim 7 wherein the second segment selector is equal to the first segment selector responsive to the operand size being 32 bit, and wherein the second segment selector is equal to the first segment selector incremented by a constant responsive to the operand size being 64 bit.
 10. The processor as recited in claim 9 wherein the execution core is configured to modify a first operating mode indication and a second operating mode indication stored in a code segment register responsive to the operand size of the third instruction.
 11. An apparatus comprising: a first storage location corresponding to a first register, the first storage location storing a first target address; a second storage location corresponding to a second register, the second storage location storing a second target address; a third storage location; and a processor coupled to the first storage location, the second storage location, and the third storage location, wherein the processor is configured, responsive to a first instruction, to: (i) select the first target address from the first storage location as a next program counter address if a first operating mode is active, and (ii) select the second target address from the second storage location as the next program counter address if a second operating mode is active, and wherein the processor is configured to store a first address of a second instruction following the first instruction in the third storage location responsive to the first instruction.
 12. The apparatus as recited in claim 11 further comprising a fourth storage location corresponding to a fourth register, a fifth storage location corresponding to a code segment register, and a sixth storage location corresponding to a stack segment register, wherein the processor is configured, responsive to the first instruction, to: (i) read a first segment selector from the fourth storage location, (ii) store the first segment selector in the fifth storage location, and (iii) generate a second segment selector from the first segment selector and store the second segment selector in the sixth storage location.
 13. The apparatus as recited in claim 12 wherein the fourth storage location further stores a third target address, and wherein the processor is configured, responsive to the first instruction, to select the third target address as the next program counter address if a third operating mode is active.
 14. The apparatus as recited in claim 11 wherein the first operating mode includes a default address size greater than 32 bits.
 15. The apparatus as recited in claim 11 further comprising a seventh storage location corresponding to a segment register, the seventh storage location storing a segment selector and at least a first operating mode indication and a second operating mode indication from a segment descriptor indicated by the segment selector, the apparatus still further comprising an eighth storage location storing an indication, wherein the processor is configured to generate an operating mode in response to the first operating mode indication, the second operating mode indication, and the indication in the eighth storage location.
 16. (Cancelled)
 17. The apparatus as recited in claim 11 wherein the processor, responsive to a third instruction, is configured to branch to the first address, the apparatus further comprising a fourth storage location configured to store a first segment selector, and wherein the processor is configured, responsive to the third instruction, to generate a second segment selector from the first segment selector responsive to an operand size of the third instruction.
 18. The apparatus as recited in claim 17 wherein the operand size of the third instruction is responsive to a prefix byte of the third instruction.
 19. The apparatus as recited in claim 17 wherein the second segment selector is equal to the first segment selector responsive to the operand size being 32 bit, and wherein the second segment selector is equal to the first segment selector incremented by a constant responsive to the operand size being 64 bit.
 20. The apparatus as recited in claim 19 wherein the execution core is configured to modify a first operating mode indication and a second operating mode indication stored in a fifth storage location responsive to the operand size of the third instruction.
 21. A method comprising: selecting a first target address from a first register as a next program counter address responsive to a first operating mode during execution of a first instruction; selecting a second target address from a second register as the next program counter address responsive to a second operating mode during execution of the first instruction; and storing a first address of a second instruction following the first instruction in a third register responsive to executing the first instruction.
 22. The method as recited in claim 21 further comprising, responsive to executing the first instruction: reading a first segment selector from a fourth register; storing the first segment selector in a code segment register; generating a second segment selector from the first segment selector; and storing the second segment selector in a stack segment register.
 23. The method as recited in claim 22 wherein the fourth register is further configured to store a third target address, the method further comprising, responsive to the first instruction, selecting the third target address as the next program counter address during execution of the first instruction responsive to a third operating mode.
 24. The method as recited in claim 21 wherein the first operating mode includes a default address size greater than 32 bits.
 25. The method as recited in claim 21 further comprising generating an operating mode in response to a first operating mode indication, a second operating mode indication, and an indication in a configuration register, wherein a segment register is configured to store a segment selector and at least the first operating mode indication and the second operating mode indication from a segment descriptor indicated by the segment selector.
 26. (Cancelled)
 27. The method as recited in claim 21 further comprising, responsive to a third instruction: branching to the first address; and generating a second segment selector from a first segment selector stored in a fourth register responsive to an operand size of the third instruction.
 28. The method as recited in claim 27 wherein the operand size of the third instruction is responsive to a prefix byte of the third instruction.
 29. The method as recited in claim 27 wherein the generating comprises: generating the second segment selector equal to the first segment selector responsive to the operand size being 32 bit; and generating the second segment selector equal to the first segment selector incremented by a constant responsive to the operand size being 64 bit.
 30. The method as recited in claim 29 further comprising modifying a first operating mode indication and a second operating mode indication stored in a code segment register responsive to the operand size of the third instruction.
 31. (New) A computer readable medium storing a plurality of instructions which, when executed responsive to a first instruction: select a first target address from a first storage location corresponding to a first register as a next program counter address if a first operating mode is active; select a second target address from a second storage location corresponding to a second register as the next program counter address if a second operating mode is active; and store a first address of a second instruction following the first instruction in a third storage location responsive to the first instruction.
 32. The computer readable medium as recited in claim 31 wherein the plurality of instructions emulate the first instruction.
 33. The computer readable medium as recited in claim 31 wherein the plurality of instructions are executed in place of the first instruction.
 34. The computer readable medium as recited in claim 31 wherein the plurality of instructions, when executed responsive to the first instruction: read a first segment selector from a fourth storage location corresponding to a fourth register; store the first segment selector in a fifth storage location corresponding to a code segment register; and generate a second segment selector from the first segment selector and store the second segment selector in a sixth storage location corresponding to a stack segment register.
 35. The computer readable medium as recited in claim 34 wherein the fourth storage location further stores a third target address, and wherein the plurality of instructions, when executed responsive to the first instruction, select the third target address as the next program counter address if a third operating mode is active.
 36. The computer readable medium as recited in claim 31 wherein the first operating mode includes a default address size greater than 32 bits.
 37. The computer readable medium as recited in claim 31 wherein a seventh storage location corresponds to a segment register, the seventh storage location storing a segment selector and at least a first operating mode indication and a second operating mode indication from a segment descriptor indicated by the segment selector, and an eighth storage location stores an indication, wherein the plurality of instructions, when executed responsive to the first instruction, generate an operating mode in response to the first operating mode indication, the second operating mode indication, and the indication in the eighth storage location.
 38. The computer readable medium as recited in claim 31 further storing a second plurality of instructions executed responsive to a third instruction, wherein the second plurality of instructions, when executed, branch to the first address, and wherein a fourth storage location stores a first segment selector, and wherein the second plurality of instructions, when executed responsive to the third instruction, generate a second segment selector from the first segment selector responsive to an operand size of the third instruction.
 39. The computer readable medium as recited in claim 38 wherein the operand size of the third instruction is responsive to a prefix byte of the third instruction.
 40. The computer readable medium as recited in claim 38 wherein the second segment selector is equal to the first segment selector responsive to the operand size being 32 bit, and wherein the second segment selector is equal to the first segment selector incremented by a constant responsive to the operand size being 64 bit.
 41. The computer readable medium as recited in claim 40 wherein the second plurality of instructions, when executed responsive to the third instruction, modify a first operating mode indication and a second operating mode indication stored in a fifth storage location responsive to the operand size of the third instruction.
 42. A computer system comprising the processor as recited in claim 1 and a peripheral device configured to communicate between the computer system and another computer system.
 43. A processor comprising: a first register configured to store a first address; and an execution core coupled to the first register, wherein the execution core is configured to select the first address as a next program counter address responsive to a first instruction, and wherein the first instruction has an associated operand size, and wherein the execution core is configured to establish, responsive to the first instruction, an operating mode in the processor for executing a second instruction stored at the first address, the operating mode selected from a plurality of operating modes responsive to the operand size of the first instruction.
 44. The processor as recited in claim 43 wherein the operand size is specified via a prefix of the first instruction.
 45. The processor as recited in claim 43 further comprising a code segment register and a second register configured to store a first segment selector, and wherein the execution core is configured, responsive to the operand size of the first instruction being a first size, to store the first segment selector in the code segment register.
 46. The processor as recited in claim 45 wherein the execution core is configured, responsive to the operand size of the first instruction being a second size, to store the first segment selector incremented by a constant in the code segment register.
 47. A computer system comprising the processor as recited in claim 43 and a peripheral device configured to communicate between the computer system and another computer system.
 48. An apparatus comprising: a first storage location corresponding to a first register, the first storage location configured to store a first address; and a processor coupled to the first storage location, wherein the processor is configured to select the first address as a next program counter address responsive to a first instruction, and wherein the first instruction has an associated operand size, and wherein the processor is configured to establish, responsive to the first instruction, an operating mode in the processor for executing a second instruction stored at the first address, the operating mode selected from a plurality of operating modes responsive to the operand size of the first instruction.
 49. The apparatus as recited in claim 48 wherein the operand size is specified via a prefix of the first instruction.
 50. The apparatus as recited in claim 48 further comprising a second storage location corresponding to a code segment register and a third storage location corresponding to a second register, the third storage location configured to store a first segment selector, and wherein the processor is configured, responsive to the operand size of the first instruction being a first size, to store the first segment selector in the second storage location.
 51. The apparatus as recited in claim 50 wherein the processor is configured, responsive to the operand size of the first instruction being a second size, to store the first segment selector incremented by a constant in the second storage location.
 52. A computer readable medium storing a plurality of instructions which, when executed responsive to a first instruction having an associated operand size: select a first address stored in a first storage location corresponding to a first register as a next program counter address; and establish an operating mode for executing a second instruction stored at the first address, the operating mode selected from a plurality of operating modes responsive to the operand size of the first instruction.
 53. The computer readable medium as recited in claim 52 wherein the operand size is specified via a prefix of the first instruction.
 54. The computer readable medium as recited in claim 52 wherein a second storage location corresponds to a code segment register and a third storage location corresponds to a second register, the third storage location configured to store a first segment selector, and wherein the plurality of instructions, when executed responsive to the first instruction, store the first segment selector in the second storage location responsive to the operand size being a first size.
 55. The computer readable medium as recited in claim 54 wherein the plurality of instructions, when execute responsive to the first instruction, store the first segment selector incremented by a constant in the second storage location responsive to the operand size being a second size. 